In the field of electronics and especially digital computers, it is desirable that a system be unaffected by either a temporary power loss or, during peak power demand periods, power "brown out". Many approaches have been used in attempting to create such a system. One approach is to have a backup generator which automatically starts when the AC power is lost. This is a costly solution and usually results in at least a few seconds of power interruption which can be tolerated for some applications but for others it could be catastrophic such as in the field of digital computers where the complete data memory could be cleared if AC power were interupted for several seconds.
In applications where a power interruption must be avoided, battery backup circuits have been developed which take over when the AC power fails. Such systems typically have a voltage sensor circuit to detect when the AC power has failed. Then, the existing power supply is electronically disconnected from the power distribution bus and the battery backup is then electronically coupled to the power distribution bus. This approach is quicker than the approach discussed above and is less expensive. However, as a complete power supply plus a full battery backup power supply is usually present, there is a significant cost to adding this type of battery backup to a typical system. In addition, when switching between the AC power line operated supply and the battery operated supply, some output voltage transitions do occur and these transitions may be a problem in certain applications such as for a solid state memory of a digital computer.
In view of the above stated difficulties, it is the principal object of the present invention to provide an uninterruptible power supply suitable for powering a solid state memory.
It is still a further objective of the present invention to provide an uninterruptible power supply that minimizes the voltage transition when the power supply is switched between AC power line operation and battery backup.
It is yet another objective of the present invention to provide an uninterruptible power supply which uses a significant portion of the circuitry for either AC power line operation or battery backup operating thereby favorably cutting the cost of the power supply and battery backup circuitry.